The invention relates to an apparatus for manufacturing a web of paper, board or similar product having a plant fiber base, and applying calcium carbonate to the web.
In the context of the present application, the term xe2x80x9cpaperxe2x80x9d is used when reference is made to any product produced on a plant fiber base such as a web of paper or board.
Generally, webs of paper and board are coated in order to enhance their printability properties. Among other things, the purpose of the coating is to improve the smoothness and whiteness of the base web and, particularly with thin base webs, to increase the opacity of the web material. Conventionally, the coating is applied to the base web surface as an aqueous suspension. There may be applied a plurality of coat layers and the application of the coat layers can be accomplished using a number of different methods. Since all the conventional methods apply the coating as an aqueous suspension, the drying of the coat forms a massive step in the coating process. Drying of the coat is an energy-intensive process and the drying of a fast moving web needs a long dryer section in order to direct a sufficient amount of drying energy to the web. Also the routing of the web to be processed is difficult, because the web cannot be guided by any members contacting it from its wet side until the coating has become adequately dry.
It is an object of the present invention to overcome the disadvantages of the above-described technology and to provide an entirely novel type of application method for coating a paper web.
The goal of the invention is achieved by forming the coating from microparticulate calcium carbonate or precursors thereof, said microparticulate coating being transferred to the web in dry form or as a suspension of a high solids content.
More specifically, the method according to the invention comprises forming a web from fibers, and treating the web with pigment particles having an average size in the range of 0.5-100 nm, most advantageously 15-25 nm.
Further, the apparatus according to the invention comprises means for supporting a web being formed, and means for forming calcium carbonate at least on the surface of the web. The means for forming calcium carbonate comprises at least means for feeding calcium hydroxide into contact with the surface of the web, and a chamber via which the web is adapted to travel and into which chamber is passed carbon dioxide containing gas in order to react carbon dioxide with calcium hydroxide so as to form calcium carbonate.
The apparatus according to the invention also comprises an electrically conducting moving wire having a first surface and a second side on a reverse side to the first side, the wire being adapted to support said web to be treated, and a means for feeding treatment material containing mineral-based components onto a web traveling on the first surface of the moving wire. At least one first electrode is positioned for forming a first potential field on the second side of the wire. At least one second electrode is disposed at a distance from the first side of the wire forming the first electrode. A high-voltage supply for elevating the voltage between the first electrode potential and the at least one second electrode high enough to establish a corona discharge in the vicinity of the second higher-potential electrode. The discharge is capable of causing an ion blast from the second higher-potential electrode to the wire forming the first electrode of the lower potential. As a result, the ion blast transports the particles of the web-treatment material existing in the space between the potentials onto the web, and assuring the adherence of the raw material to the web.
The invention offers significant benefits.
By virtue of the invention, a web can be coated without using any water in the coating furnish, whereby also the web need not be dried during any step of the coating process and no equipment is required for preparing an aqueous coating mix. The overall length of the coater can be made shorter and its construction may be simplified. Because a major fraction of the energy consumed at the coater is used for drying the aqueous coating suspensions the invention offers significant savings in the specific energy consumption of coated paper grades. The invention may be combined with a dry base web-formation method (cf. Finnish Patent Application No. 973775), thus permitting the entire papermaking process to be carried out without any drying steps or, alternatively, using a minimal amount of water and drying. During web formation, calcium carbonate or its precursors can be fed into the fiber mass, whereby also the filler is introduced into the web. As known, calcium carbonate particles or the formation thereof in the web improves the interfiber bonding, whereby the web becomes stronger than a web containing no fillers.
By virtue of the invention, the paper surface can be provided with a coat of superior hiding power and smoothness. This kind of paper offers excellent printability. Moreover, coat defects such as streaking, orange peel patterning, uncoated patches and the like typically occurring in conjunction with blade, film-transfer and other known coating methods are eliminated. The dust of coating material can be effectively adhered to the web, whereby the soiling of the operating environment and equipment is smaller than with conventional wet coating methods. The method can be combined with known coating methods, thus permitting a dry coating to be applied on a wet coat and vice versa. The method can cost-effectively produce thick coats, because there is no need for intermediate drying between the application of successive coat layers and the thickness of a coat layer is limited only by the adherence of the coating to the web surface. To obtain heavy coat weights, it may be necessary to use binders that are applied to the web simultaneously with the coating or, during a separate step, under or over the coat. The calcium carbonate particles can be prepared in a separate process or coating equipment, or even directly onto the web.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.